1. Field of the Invention
The present invention relates to a driver for an alternating current thin film electro luminescence (AC TFEL) device, and more particularly to a circuit for driving an AC TFEL device, capable of generating positive and negative voltages required to drive the AC TFEL device which is a positive solid luminescence device of a planar display device constituting an important part of a hanging television receiver or a portable computer and exhibits a superior endurance against surroundings, a large view angle and a high response speed.
2. Description of the Prior Art
FIG. 1 is an electrically equivalent circuit of an AC TFEL device. As shown in FIG. 1, an AC voltage is applied to the AC TFEL device, for driving of the AC TFEL device. For driving the AC TFEL device, actually, DC pulses of positive polarity and negative polarity are alternatingly applied to the AC TFEL device. For luminescence of the AC TFEL device, the pulses should exceed the threshold voltage Vth of about 135 to 175 V. Brightness of the AC TFEL device is adjusted by a modulation voltage Vm of about 40 to 60 V. The AC TFEL device has a behavior that it emits light when it receives voltage pulses of opposite polarities.
In a display device using such an AC TFEL device, a matrix drive system is generally utilized. As a method for driving the AC TFEL device, a refresh drive method and a symmetric drive method are mainly used. The refresh method is the method wherein scanning pulses are applied row by row and after the scanning of one frame is completed, refresh pulses having an opposite polarity to that of the scanning pulses are applied to all rows simultaneously so as to discharge the charge accumulated in each pixel. The refresh method provides a characteristic that for one frame, light is emitted two times, namely, one time when the scanning pulses are applied and the other time when the refresh pulses are applied.
However, the refresh drive method encounters a problem of residual DC voltage difference among pixels because the interval of pulses applied to each pixel is asymmetric. Due to such a problem, the refresh drive method has a disadvantage that a latent image is generated by the lapse of use time.
The symmetric drive method has been proposed to solve the above-mentioned problem encountered in the refresh drive method. This method is the method wherein the polarity of scanning pulses is changed every time when a change of frame occurs. In accordance with the symmetric drive method, the interval of pulses applied to the pixels is uniform. This solves the problem of residual DC voltage difference encountered in the refresh drive method. However, the symmetric drive method has a disadvantage of a degradation in brightness, as compared with the refresh drive method. This is because for one frame, light is emitted only one time in accordance with the symmetric drive method.
Meanwhile, there has been also proposed a scan inversion symmetric drive method for solving the problem of residual DC voltage difference and yet maintaining a brightness as obtained in the refresh drive method. This method is disclosed in European Paten No. EP 295852.
The scan symmetric drive method is similar to the refresh drive method, except that the scanning sequence of row electrodes is inverted every time when a change of frame occurs. In accordance with the scan inversion symmetric drive method, for a frame, scanning pulses are applied in a sequence from the first row to the last row. For the next frame, scanning pulses are applied in a sequence from the last row to the first row. In accordance with the scan inversion symmetric drive method, residual DC voltages respectively applied to the pixels become uniform by inverting the scanning sequence of row electrodes for every frame change. The refresh drive method and the scanning sequence inversion drive method require an open-drain circuit at the side of rows and a push-pull circuit at the side of columns. On the other hand, the symmetric drive method requires push-pull circuits at both the side of rows and the side of columns.
Now, the basic concept of the refresh drive method upon driving an AC TFEL matrix will be described.
Cross areas of rows and column electrodes serve as electroluminescence (EL) pixels. Upon scanning of row electrodes, a voltage pulse of -Vth is applied to the row electrodes. To column electrodes, a modulation voltage Vm is applied for "ON" pixels, while zero voltage is applied for "OFF" pixels.
As a result, the pixels to be at the ON state receive the voltage of -(Vth+Vm), so that they may emit light. On the other hand, the pixels to be at the OFF state receives only the voltage of -Vth, so that they may not emit light. After completion of the scanning of all row electrodes for one frame, a voltage of +(Vth+Vm) is applied to all the row electrodes as a refresh pulse, while zero voltage is applied to all the column electrodes.
At this time, all pixels receive the voltage of +(Vth+Vm). This means that the voltage pulse having the opposite polarity to that upon the scanning is applied. It is noted that by virtue of the charge movement amount of the pixels upon refreshing, the pixels which were at ON state by the voltage of -(Vth+Vm) applied thereto upon the scanning are still maintained at ON state, while the pixels which were at OFF state by the voltage of -Vth applied thereto upon the scanning are still maintained at OFF state.
FIG. 2 is a circuit diagram of a matrix driving circuit operating in accordance with the conventional refresh drive method and scan inversion symmetric drive method. As shown in FIG. 2, the circuit includes three voltage supply units 1, 2 and 3, and a voltage selection circuit 4 adapted to select voltages respectively required for a column-driving integrated element upon refreshing and scanning.
In driving of the conventional TFEL device, three voltages, +(Vth+Vm), -Vth and +Vm should be used, in addition to a voltage for driving a logic circuit, in order to accomplish the AC TFEL refresh drive method and the scan inversion symmetric drive method. For supplying these voltages, a voltage supply circuit which is constituted by a switching regulator circuit is required. However, the use of such a voltage supply circuit results in a difficulty to achieve compactness and lightness of electronic devices. Furthermore, there is a problem of a loss of energy in a power supply unit. The switching regulator also causes a switching noise problem that makes it difficult to embody desired electronic devices.